Ionospheric scintillation produces strong disruptive effects on global navigation satellite system (GNSS) signals, ranging from degrading performances to rendering these signals useless for accurate navigation. The current paper presents a novel approach to detect scintillation on the GNSS signals based on its effect on the ionospheric-free combination of carrier phases, i.e. the standard combination of measurements used in precise point positioning (PPP). The method is implemented using actual data, thereby having both its feasibility and its usefulness assessed at the same time. The results identify the main effects of scintillation, which consist of an increased level of noise in the ionospheric-free combination of measurements and the introduction of cycle-slips into the signals. Also discussed is how mis-detected cycle-slips contaminate the rate of change of the total electron content index (ROTI) values, which is especially important for low-latitude receivers. By considering the effect of single jumps in the individual frequencies, the proposed method is able to isolate, over the combined signal, the frequency experiencing the cycle-slip. Moreover, because of the use of the ionospheric-free combination, the method captures the diffractive nature of the scintillation phenomena that, in the end, is the relevant effect on PPP. Finally, a new scintillation index is introduced that is associated with the degradation of the performance in navigation.

Fast Precise Point Positioning (Fast-PPP) provides Global Navigation Satellite System corrections in real-time.
Satellite orbits and clock corrections are shown to be accurate to a few centimeters and a few tenths of a nanosecond
which, together with the determination of the fractional part of the ambiguities, enable global high-accuracy positioning with undifferenced Integer Ambiguity Resolution. The new global ionospheric model is shown to provide corrections accurate at the level of 1 Total Electron Content Unit over well-sounded areas and Differential Code Biases at the level of tenths of a nanosecond.
These corrections are assessed with permanent receivers, treated as rovers, located at 100 to 800 kilometers from the reference stations of the ionospheric model. Fast-PPP achieves decimeter-level of accuracy after few minutes, several times faster than single- and dual-frequency ionospheric-free solutions, using a month of Global Positioning System data close to the last Solar Maximum and including equatorial rovers.

Applying a methodology developed and tested in
previous studies, the contribution from the ionospheric and
plasmaspheric regions to the total electron content (measured by
ground receivers) is analyzed. The method is based in the
electron density profiles retrieved from radio occultations
observed with low Earth orbit satellites, combined with an
accurate empirical modeling of the topside-ionosphere electron
density. The results of a climatological study of the fractional
electron content from the ionospheric region are presented for a
year of low solar activity. It is shown that a simple parametric
model can be used to reproduce the electron content variations in
the ionosphere and the plasmasphere between sunrise and
midday, the period of the day showing the largest electron
content variability.

The research of this paper-based dissertation is focused on the Fast Precise Point Positioning (Fast-PPP) technique. The novelty relies on using an accurate ionosphere model, in combination with the standard precise satellite clock and orbit products, to reduce the convergence time of state-of-the-art high-accuracy navigation techniques from approximately one hour to few minutes.
My first contribution to the Fast-PPP technique as a Ph.D. student has been the design and implementation of a novel user navigation filter, based on the raw treatment of undifferenced multi-frequency code and carrier-phase Global Navigation Satellite System (GNSS) measurements. The innovative strategy of the filter avoids applying the usual ionospheric-free combination to the GNSS observables, exploiting the full capacity of new multi-frequency signals and increasing the robustness of Fast-PPP in challenging environments where the sky visibility is reduced. It has been optimised to take advantage of the corrections required to compensate the delays (i.e., errors) affecting the GNSS signals. The Fast-PPP corrections, and most important, their corrections uncertainties (i.e., the confidence bounds) are added as additional equations in the navigation filter to obtain Precise Point Positioning (PPP) in few minutes.
A second contribution performed with the new user filter, has been the consolidation of the precise ionospheric modelling of Fast-PPP and its extension from a regional to a global scale. The correct use of the confidence bounds has been found of great importance when navigating in the low-latitude areas of the equator, where the ionosphere is difficult to be accurately modelled. Even in such scenario, a great consistency has been achieved between the actual positioning errors with respect to the formal errors, as demonstrated using similar figures of merit used in civil aviation, as the Stanford plot.
A third contribution within this dissertation has been the characterisation of the accuracy of different ionospheric models currently used in GNSS. The assessment uses actual, unambiguous and undifferenced carrier-phase measurements, thanks to the centimetre-level modelling capability within the Fast-PPP technique. Not only the errors of the ionosphere models have been quantified in absolute and relative terms, but also, their effect on navigation.

Two high-precision positioning techniques currently offer accur acy at the centimetre level: Real-Time Kinematics (RTK) and Precise Point Positioning (PPP). Both methods use carrier-phase measurements, 2 orders of magnitude more prec ise than pseudoranges. Classical single-baseline RTK (appeared in the 80’s) uses a nea rby reference station to compensate most of the delays (i.e., errors) affecting GNSS sig nals. RTK achieves centimetre-level of accuracy in seconds after the Double Differ ences of the carrier-phase ambiguities are fixed to integers. The drawbacks of RTK are: i) the bandwidth and continuity requirements to disseminate the measurements from th e reference receiver to the user and ii) the maximum distance to the reference station, which can range from 10-20 km (depending on the ionosph eric activity) to 50 km using Network-RTK. PPP (defined in the 90’s) overcomes the RTK limitations with du al-frequency measurements and orbit and clock products precise to a few cent imetres. PPP products require less bandwidth than RTK, with less continuity constrain s and allow world-wide coverage. However, PPP requires almost 1 hour to convergence th e un-differenced carrier-phase ambiguity estimation from the noisy pseudorange. This initialization is not acceptable in most professional kinematic applications (e.g. su rveying, farming) that usually rely on RTK. Recent improvements to PPP are: (i) the or bit and clock corrections are sent to users i n real-time, (ii) the user can f ix the carrier ambiguities in undifferenced mode, improving the accuracy, (iii) the multi-con stellation context. In this presentation we will review the main features of the Hi gh Accuracy Positioning techniques, from RTK to PPP. In particular we will address some the large convergence time of PPP and the lack of integrity in the user solution. Fin ally we will show how a World-Wide Ionospheric Model for Fast-PPP reduces the convergence time in PPP and, also, enables High-Accuracy navi gation with a single frequency receiver.

South East Asia is growing at an impressive pace with its GDP having increased by about 350% in ten years , ri sing from 650 Billion $ in 2002 to about 2300 Billion $ in 2012. Regional governments actively promote infrastructure, logistics and service development to create a favourable environment fo r sustainable growth. Within this framework, GNSS applications play a vital role. In particular, the increasing demand for better services , in both the public and private sector , and logistics is going to require increasingly reliable and trusted GNSS appl ications. South East Asia has the highest multi - GNSS coverage in the world, and is therefore the ideal place to test and compare performance and opportunities offered by the different g lobal n avigation s atellite s ystems. On the one hand, an active promotio n of E U GNSS (EGNSS) technology in this region, rising awareness on its main features while on the other hand facilitating the linking of European enterprises with South East Asian GNSS stakeholders, are extremely important so as to establish and maintain global European scientific and industrial leadership in this crucial sector. Indeed, it is also fundamental to encourage the penetration of EGNSS industry in to the South East Asian market taking into account all potential applications of EGNSS. Therefore, the BELS project exploits the opportunities presented by the NAVIS Centre, an International Collaboration Centre for Research and Development on Satellite Navigation Technology in South East Asia based in Hanoi, Vietnam, to promote visibility and to raise awareness o f EGNSS technology in the region. The next three years will be crucial for pav ing the way for Galileo services, both for European companies that can enter a new growing market and for the South East Asian countries that can discover the capabili ties of the EGNSS technology. Consequently, BELS facilitates and supports the visits of European companies for such purposes a s carrying out tests in the NAVIS Centre and hence to assist them in getting ready for the global GNSS applications market .

The main objective of this work is to present a methodology to assess the accuracy of any ionospheric model used in Global Navigation Satellite System (GNSS) applications. A number of global and regional models (both in realtime and post-process) will be analyzed during the entire 2014, i.e. near to the last Solar Cycle Maximum, to identify seasonal characteristics. The new method uses as reference values the unambiguous and undifferenced geometry-free combination of carrier-phase measurements from a worldwide distribution of receivers. The differences between the Slant Total Electron Contents (STECs) of the model and the measurements are fit to constant hardware delays: a receiver plus a satellite Differential Code Bias (DCB). Once such DCBs are estimated, the post-fit residual of the adjustment to the reference values is computed. It is shown that this residual is a very suitable metric to represent the error of any ionospheric model tailored for GNSS-based navigation. Any miss-modeling present in the STECs predictions which cannot be represented by a constant parameter per station and a constant per satellite degrades the user positioning. The assessment includes the comparison of the 3D navigation error of some permanent stations, being processed in singlefrequency as kinematic rovers, using different ionospheric corrections and precise satellite orbits and clocks.

We introduce a methodology to extract the separate contributions of the ionosphere and the plasmasphere to the vertical total electron content, without relying on a fixed altitude to perform that separation. The method combines two previously developed and tested techniques, namely, the retrieval of electron density profiles from radio occultations using an improved Abel inversion technique and a two-component model for the topside ionosphere plus protonosphere. Taking measurements of the total electron content from global ionospheric maps and radio occultations from the Constellation Observing System for Meteorology, Ionosphere, and Climate/FORMOSAT-3 constellation, the ionospheric and plasmaspheric electron contents are calculated for a sample of observations covering 2007, a period of low solar and geomagnetic activity. The results obtained are shown to be consistent with previous studies for the last solar minimum period and with model calculations, confirming the reversal of the winter anomaly, the hemispheric asymmetry of the semiannual anomaly, and the existence in the plasmasphere of an annual anomaly in the South American sector of longitudes. The analysis of the respective fractional contributions from the ionosphere and the plasmasphere to the total electron content shows quantitatively that during the night the plasmasphere makes the largest contribution, peaking just before sunrise and during winter. On the other hand, the fractional contribution from the ionosphere reaches a maximum value around noon, which is nearly independent of season and geomagnetic latitude.

Fast precise point positioning (Fast-PPP) is a satellite-based navigation technique using an accurate real-time ionospheric modeling to achieve high accuracy quickly. In this paper, an end-to-end performance assessment of Fast-PPP is presented in near-maximum Solar Cycle conditions; from the accuracy of the Central Processing Facility corrections, to the user positioning. A planetary distribution of permanent receivers including challenging conditions at equatorial latitudes, is navigated in pure kinematic mode, located from 100 to 1300 km away from the nearest reference station used to derive the ionospheric model.
It is shown that satellite orbits and clocks accurate to few centimeters
and few tenths of nanoseconds, used in conjunction with an ionosphere with an accuracy better than 1 Total Electron Content Unit (16 cm in L1) reduce the convergence time of dual-frequency Precise Point Positioning, to decimeter-level (3-D) solutions. Horizontal convergence times are shortened 40% to 90%, whereas the vertical components are reduced by 20% to 60%. A metric to evaluate the quality of any ionospheric model for Global Navigation Satellite System is also proposed. The ionospheric modeling accuracy is directly translated to mass-market single-frequency
users. The 95th percentile of horizontal and vertical accuracies is shown to be 40 and 60 cm for single-frequency users and 9 and 16 cm for dual-frequency users. The tradeoff between the formal and actual positioning errors has been carefully studied to set realistic confidence levels to the corrections.

The ionosphere plays an important role in satellite-based navigation, either in standard navigation, with single frequency mass-market receivers, or in precise navigation, with dual frequency receivers.
In this work, the requirements of a real-time ionospheric model suitable for GNSS applications are explored, in terms of accuracy and confidence bounds. Key factors for an ionospheric determination better than 1 Total Electron Content Unit (TECU) (16 centimeters in L1) are shown to be whether the model has been derived using an ambiguity-fixing strategy and the number of layers used to reproduce the ionospheric delay. Different models are assessed both in mid-latitudes and equatorial regions, near the Solar Cycle maximum.
It will be shown how dual-frequency users take benefit from a precise modelling of the ionosphere. If accurate enough, the convergence of the navigation filter is reduced to achieve high accuracy positioning quickly, (i.e., the Fast Precise Point Positioning technique). Satellite orbits and clocks computed for Fast-PPP will be shown to be accurate to few centimeters and few tenths of nanoseconds, respectively.
Single-frequency users correct its measurements with the predictions provided by any ionospheric model. Thence, the accuracy of the Fast-PPP ionospheric corrections is directly translated to the measurements modelling and, consequently, to the user solution.
Horizontal and vertical 95% accuracies are shown to be better than 36 and 63 centimeters for single-frequency users and 11 and 15 centimeters for dual-frequency users. The assessment is done for several locations, including the equatorial region, for a month of data close to the last Solar Maximum. The trade-off between the formal and actual positioning errors has been carefully studied by means of the Stanford plots to set realistic confidence bounds to the corrections.